Modulation system for radio transmitters



June 6, 1939- E. scHULzE-HERRINGEN 2,161,320

MODULATION SYSTEM FOR RADIO TRANSMITTERS Filed April 8, 1937 5 Sheets-Sheet l m" VTI/p2 CAMP/5e impur/2? MODULATION SYSTEM FOR RADIO TRANSMTTTERS Filed April 8', 1937 s sheets-sheet 2 June 5, v1939. E. scHULzE-HERRlNGEN 2,161,320

MODULATION SYSTEM FOR RADIO TRANSMITTERS Filed April 8, 1957 3 Sheets-Sheet I5 Patented June 6,1939 a i o fy f, UNITAEKD PATENT OFFICE vMonlfmli'rIoN SYSTEM Fon RADIO n l' y cru'mANsivn'rTERs Erich 'Schulze-Herringen, Berlin-Tempelhof,

' Germany, assignor to C. Lorenz Aktiengesellschaft, Berlin-Tempelhof (Germany), Lorenz- Weg,y a company -ApplicationAprl 8, 1937, Serial No. 135,721 v,- M In Germany April 9, 1936 7 Claims.` (Cl. 179-171) In vorder to ,modulate l highl frequency trans- Fig. 1 is a diagrammatic View of a known transmitters methods varaknownin which a saving of mitting system having separate carrierand side energy is effected by separating carrierfrequency band amplifiers. Fig. 2 is a View similar to Fig.

and side *bandsv from eachother, these two then 1 and illustrates the fundamental idea of the in- 5 being amplified and` sentout separately. l Such vention. Fig. 3 is a graphic representation that `5 methods have thefdisadvantage that lthe carrier serves to explain this idea. Fig. 4 is a wiring amplifier may-operateat a highl efficiency, only diagram of a device that forms part of the arthe side band amplifier having to operate at a rangements shown in Figs. 1 and 2 and acts to low efficiency. In arrangements of thisk kind, separate the side bands from the carrier. Fig. 5

however, it is not possible'to ycause the two amis a graphic representation which serves to ex- -10 pliers to act on an antenna common rto them, plaink the function of amplifiers included in the because due to a reaction of impedance variations arrangement shown in Fig. 2. Fig. 6 is a graphic that occur in the antenna the highly efficient representation that relates to the function of an carrier amplier` will, deliver yless power if the amplifier adapted for use in connection with l5 modulation degree Vis increasing. VIf such dethe arrangement shown in Fig. 2. Fig. '7 is a 15 crease in power loutput were to be avoided then graphic representation which serves to explain the ycarrier amplifier rwould have to operate at distortions which occur in amplifiers of this kind. lower efficiency, whereby however the desired Fig. 8 is a graphic representation that relates to saving of energy would get lost. Therefore, as the function of amplifiers used in arrangements stated before, the two amplifiers have been caused as provided by the invention. Fig. 9 is a wiring 20 to act on separate antennae which are as far as diagram of these arrangements. possible neutralized with regardto each other, Reverting to Fig. 1, in which a known arso that there shall be no coupling effect between rangement is shown, S denotes the control stage, them. To such end the antennae are spaced I the stagein which the side bands are separated apart by a sufliciently large distance, or addirfrom' the carrier, the carrier being suppressed 25 tional means are kprovided for establishing an here. II designates the side band amplifier, El opposing rcoupling effect.v Thus `the impedance the final stage of the side band amplifier, Ai the variations of that* antenna on which the side lantenna, which emits the side band or the side band amplifier is actingr are prevented from rebands. 'Ihe modulation is effected by the moduactingupon the output circuit of the carrier amlation amplifier M connected in advance of stage 30 plifier. This, however, has the disadvantage that I. v The carrier is derived from stage S, is then a double antenna plant is used, thus involving emplified in the carrier amplifier III and is then great first .cost and considerable space requireconveyed over the nal stage E2 to antenna A2. ments. In Fig. 2, which illustrates the inventive idea,

` The invention proposes to cause carrier amlthe reference characters are as far as possible 35 Dlier and Side bandamplier to act on a comthe same as used in Fig. 1. The two nal ammon or single utilization circuit and also prof yplifiers El, E2 are here arranged to act on an Vides means adapted to avoid the s'a'id reaction, antenna common to them. In the lead from that is,` the influence which the side band am- E2 to antenna A a member CI is interposed that 0 plier has upon the carrier amplifier. These acts to rotate the phase of the carrier by 90. 40 means reside in a reversal of the impedance varia- This phase rotation or phase shift, whenever the tions that occur in the saidcommon utilization impedance of the antenna circuit increases under rcircuit andvin the final stage of thecarrier am- `the action of the final stage of the side band am'- plifien'thatisto say, if the impedance ofthe utilplifer El, causes the impedance at the output ization circuit is increasing then the impedance `side of E2 to decrease, and vice versa. If the 45 of the output circuit of the carrier amplifier is youtput impedance of E2 decreases then the power decreased. This iS effected by interposing beoutput of this stage will increase, since the optween the utilization circuit and the output side eration takes place at high efficiency. In order, of the carrier amplifier a member of the length however, to ensure phase correctness of the car- \/4, this member being capable of effecting such rier when supplied to the antenna the phase ro- 50 impedance reversal. i denotes the wavelength. tation effected in Cl must be compensated. This From the following description the state of the is done in accordance with a feature of the inart and the idea of the invention will be undervention by inserting a member C2 before the nal stood,v reference being had to the accompanying jstage,-this member acting to rotate the phase by ,drawings in which l, 90 in the opposite direction. It is immaterial 55 compensated by amplifier III'. In this case of course C2 would have to be interposed between S and III. Furthermore, C2 may be constituted by a chain system of any construction. course be of the same construction as described with regard to C2. As, however, the output at Cl is very great it will be convenient not to construct CI as a phase bridge or chain systeml but to interpose an energy line, such for example as aconcentric high frequency cable, of such length that the phase rotation by or so called phase quadrature shall be obtained in this way. Such cable may for instance be wound on a drum or may be'bent on its way to the antenna coupling cabin. rIlhe first arrangement has the advantage that only a single energy line leading from the transmitter to the antenna is necessary.

FigrS shows how the output delivery of the final stage E2 increases with decreasing output impedance. The output efiiciency N is here plotted with respect to the output impedance Z. The top of the curve is the limit between the high and low efficiency states. On the left-hand side of the dotted line y the operation is effected at low efli'ciency while on the right-hand side thereof it takes place at high efiiciency. Since, as stated, the carrier amplifier is to operate at a high efiiciency, the working point is located in the region of high efliciency, for instance at P. To this working point a certain output efciency N l corresponds, the output impedance being of the value ZI. By diminishing ZI, for instance to Z2, the efficiency increases to N2 while the working point is displaced from P to P. This surplus eiciency with decreasing impedance is well known per se. However, no means have been proposed so far to convert an increase of the impedance of the antenna circuit into a decrease of the output impedance of the amplifier. In accordance with the invention such means is constituted by the member Cl. The construction of the prestages II, III of the side band amplifier lor carrier amplifier, respectively, is not of interest here. Stage I in which the side bands are separated from the carrier may be for instance in push-pull connection in a well known manner. To the input side of this push-pull stage the modulating voltage is supplied in phase opposition while the carrier is supplied thereto in co-phasal relation. In the case of no modulation the arrangement is balanced, that is to say, in the anode circuit of the push-pull stage the carrier currents will 'annul each other. If modulated, however, the balance is relieved in accordance with the modulating voltages, and it is so in such manner "that differential high frequency currents will flow inthe anode circuit of the push-pull stage which correspond with the modulating voltagesv impressed upon the grids thereof. The fundamental arrangement of such a push-pull stage is shown in Fig. 4.

.In the arrangement represented in Fig. 4 the modulating voltage is supplied over terminals I,

2 while over. terminals 3, 4 the carrier is supplied.

it thefoutput terminals 5, 6 only the side bands are derived.A

Cl may ofA rtotal current.

l.the curves b, c1 of Fig. 5 are obtained, by representing the carrier and c the side bands. The carrier;l b,Fig. 5, is still further amplified in the carrierV amplifier III, Fig, 2, while the side bands c are so in the side band amplifier II.

Inv order'tor save energy the final stage of the side band amplifier,. i. e., the stage operating in a state of low tension, may be constructed as an amplifier in Class B connection. Fig. 6 discloses the conditions which relate to such a Class B amplifier.y The working point P is located in the lowerbend of the idealized characteristic curve K. The side bands Sv are conducted to the grid of the stage and are after amplification derived from the anode circuit in an undistorted condition. With arrangements of this kind the at-rest current initsunmodulated state is equal to zero. However, the real characteristic curve of an amplifier tube has no sharp bend at its lower end, that is to say, does notl join the zero line in the manner represented inrFig. 6-but joins this by a curvature, as shownV in Fig. '7. As a result, behind the final stage distortions arise, which depend upon the shape of the curvature. Fig 'I shows how suchdistortions originate. If the working point is made to be point P of the real characteristic K then in consequence of the curvature no undistorted curves are obtained at the output side of the amplifier, but the resultant curves are flattened as illustrated at a. If the working point is made to be point P', that is, the point where the characteristic ceases to be straight, then inthe first vplace an at-rest current J is produced. That part of the side bands which was to be suppressed is likewise amplified and therefore infits turn entails a distortion. The curves belonging to working point P are designated b.

In order to avoid these non-linear distortions a further feature of the invention proposes to insert a single-way rectifier into the grid circuit of the Class B amplifier, such for example as a diode that acts to cut away one half of the side bands. In this case the working characteristic of the rectifier shouldbefmade as straight as possible. Fig. 8 shows the mode of action of such an arrangement. The working point is point P, located at the end of the straight portion of the characteristic K. Owing to the rectification effected by the diode, only curve lines of the kind represented in Fig; 8 are conveyed to the grid circuit of the tube, the other curve lines, which are in the nature of mirror refiections to the former, being cut away. Since point P is not located in the zero line, again an at-rest current J results which, however,- is not troublesome as stated before, being small as compared with the From the anode circuit of the amplifier a true or distortionless reproduction of the curves supplied to the arrangement may now be derived after amplification.

Fig. 9 by Way of example shows a wiring diagram of the iinal stage of the Class B amplifier. 'I'he working'point P is adjusted by regulating the grid bias G. The diode D in the grid circuit acts to cut' awayr one half of the curves conveyed over transformer T. By tuning; the anode circuit of the stage the carrier Wave'may be filtered out so that the two side bands are reproduced in their original form.

In accordance with still another feature of the invention the working point is displaced in the customary manner into the point where the characteristic curve of the Class B amplifier and the zero line intersect, and the working characteristic of the rectifier is so chosen that distortions which are due to the curvature of the amplifier characteristic shall be compensated by the curvature of the rectifier characteristic.

The novel arrangement thus ensures that when the carrier amplifier is operating at high efficiency an impedance increase that occurs in the utilization circuit does not decrease the power output but acts to increase it. In this way a saving in the expense for place requirements and installation is made as compared with prior arrangements of the kind operating likewise at high efficiency, since one of the usual antenna arrangements is now dispensed with. A surplus expenditure resides only in the provision of an additional length of energy line, namely the M4 length necessary for effecting the described phase shift,this expenditure however being immaterial over the said advantage of the novel arrangement.

Owing to the invention it is possible to cause carrier amplifier and side band amplifier to act on a common or single utilization circuit. Therefore, in accordance with a feature of the invention the method of separate amplification may now be used in connection with radio transmission systems of the kind operating with conductors. For the reasons stated before this cannot be achieved with the means that have been provided in prior systems.

What is claimed is:

1. In a radio transmitting system having means for obtaining and separating side bands from a carrier, an amplifier for the carrier frequency, a high eflciency amplifier for the side bands, and a single utilization circuit, means for coupling said amplifiers to said utilization circuit, and means for reversing in the output circuit of the carrier amplifier impedance-variations occurring in said utilization circuit.

2. An arrangement defined in claim 1, wherein the side band amplifier is of the Class B type having a vacuum tube with anode, cathode and grid circuit, comprising a one-way rectifier included in the grid circuit and means adapted for displacing the working point into the starting point of the straight part of the characteristic curve of the Class B amplifier.

3. An arrangement defined in claim l, wherein the side band amplifier is of the Class B type having a vacuum tube with anode, cathode and grid circuit, comprising a one-way rectifier included in the grid circuit and having a straight characteristic curve.

4. In an arrangement defined in claim l, wherein the side band amplifier is of the Class B type having a vacuum tube with anode, cathode and grid circuit, comprising a one-way rectifier included in the grid circuit and means adapted for displacing the Working point into the point of intersection of the characteristic curve and zero line, the characteristic curve of this rectifier being such that distortions caused by the characteristic curve of the Class B amplifier are compensated.

5. An arrangement according to claim 1, means inserted between the utilization circuit and the output circuit of the carrier frequency amplifier for quadrature phase shifting the high frequency output of said amplifier, and a second means producing the same angular phase shift to compensate for the first mentioned phase shift.

6. An arrangement according to claim 1, means inserted between the output of the carrier amplifier and said utilization circuit for shifting the phase of the carrier, said means comprising a transmission line of one-quarter Wave length.

7. An arrangement according to claim 1, wherein the side band amplifier is of the Class B type.

ERICH SCHULZE-HERRINGEN. 

